The ability of individual cells to adhere and coalesce into distinct tissues is a major feature of multicellular organisms. Cell-cell adhesion is largely mediated by a protein complex that projects from the cell surface to form a structural Velcro that holds cells to one another. This complex is comprised of a transmembrane cadherin component that mediates Ca++-dependent homophilic recognition, and associated catenins that link cadherins to the underlying cytoskeleton. Epithelial (E)-cadherin is the prototypic classical cadherin present on epithelia. Reduction or loss of E-cadherin has been observed in numerous human epithelial cancers and is considered a key rate-limiting step in tumor metastasis. The cytoplasmic domain of classical cadherins binds the dual function adhesion/transcriptional co-activator protein, ?-catenin, which in turn binds the F-actin binding protein, ?-catenin, effectively coupling adhesion to the actin cytoskeleton. It has been known for over a decade that cytoplasmic tail of E-cadherin is robustly phosphorylated in the ?-catenin binding region and that this phosphorylation increases the affinity for ?-catenin in vitro. However, the function and regulation of E-cadherin phosphorylation in vivo remain poorly defined. We find that E-cadherin phosphorylation is required for effective binding to ?-catenin binding in vivo, suggesting the hypothesis that modulation of E-cadherin phosphorylation directs changes in cell-cell adhesion. We seek to determine the kinase (or kinases) that phosphorylates the cytoplasmic tail of E-cadherin and the specific amino acids that are phosphorylated using a dsRNA Drosophila cell screen and mass spectrometry (Aim 1). In Aim 2 we seek to determine the contribution of E-cadherin phosphorylation and ?-catenin binding on its trafficking to and endocytosis from the plasma membrane. Altogether, these aims will lead to an understanding of how E-cadherin phosphorylation is regulated and will define its role in cell-cell adhesion, two questions that are broadly relevant to both normal epithelial integrity and tumor metastasis.